MeasurementBlog

To continue: Chapter 2.

Optical Pyrometers were used very much in the development, design and manufacturing of electronic vacuum tubes. Very few are made today, but even in those uses, there were severe limitations to “Opticals”. Not the least was the fact that most of the measurements were made on heated components within a vacuum.

That meant the user had to correct for the spectral emissivity of the heated component as well as correct for the loss in apparent emitted radiation due to the transit of the thermal radiation through the glass envelope of the tube walls. Some smart scientists and engineers knew how to do the corrections and since they were almost always the same materials in any given tube design, the correction was relatively easy to apply.

Back in the electronic vacuum tube days, of course, we didn’t have PDAs and pocket calculators, so the corrections were often set out in a chart or table. Even simpler, was to build the correction into the process parameters and target the “Effective Spectral Radiance Temperature” rather than the true temperature.

Using spectral radiance temperature was a great simplification because the principle calibration sources for Opticals were flat filament calibration lamps that were composed of a flat tungsten filament lamp with a certified spectral radiance temperature versus filament current property. There was a bit more to it than that, but that was the essence of calibration traceability for the Pyrometer Calibration Lamps or Standard Lamps, as they were often called.

Some of the problems implied by this calibration and application shortcut were the spectral emissivity correction questions, that arose often, such as:

What is the spectral radiance temperature? How does it vary with wavelength?
How does it depend on the materials of the tube especially if they are not the same tungsten as the flat filament calibration lamp?
What if they are coated like with an electron-emission enhancement material like thorium oxide?
Why can’t we just reference everything to the true temperature and forget about the complexities of spectral radiance temperature?

NOTE: I am reasonable careful to use the term, spectral emissivity, not just emissivity. There is a big difference between the two and if you do not understand them, visit the E-missivity Trail at About Temperature Sensors for more complete details. Note, too, that because spectral radiance temperature depends upon the wavelength (or effective wavelength) of the measuring device that it can be thought of as a lot like beauty-its value depends on the “eye” of the measurement device.

The true temperature read by any radiation thermometer (and an Optical Pyrometer is an example of one kind that operates in the visible region of the Optical Spectrum) can be related to the spectral radiance temperature by a relatively simple equation, if the Pyrometer effectively has a single waveband and there is an insignificant amount of reflected radiation from the object within the measurement waveband, (or problems like absorptions or emissions within the sight path between the object and the instrument).

(Turns out that an effective single waveband can be applied to just about any radiation thermometer if one is careful and has an error tolerance of about 1/2 to 1% of absolute temperature see ASTM STP 895 “Applications of Radiation Thermometry” or any modern book dealing with Radiation Thermometry ).

Anyhow, the radiance temperature equation for any radiation thermometer that works under Wein’s Approximation [(C2/lambda x T)<<1], where there are no significant reflected radiation effect: is:

Trad = (spectral emissivity) xK x e^-(c2/[lambda x T]),

By a little algebra manipulation, it can be shown that the Trad and T can be shown related as:

(1/T -1/Trad)= (lambda/c2) x Ln {(spectral emissivity)},

Where:

Trad is the radiance temperature in Kelvin,
T is the true temperature in Kelvin,
c2 is the second Planck radiation constant approximately 14388 micrometer K,
lambda is the effective single wavelength, in micrometers (microns) and,
and e is the base of the natural logarithm, 2.718281… and,
Ln is the natural logarithm, i.e. If a= e^(b) then Ln a = b.

Bottom Line:

You need and equation or graph PLUS a knowledge of the spectral emissivity of an object to get to the true temperature (and then of course you need to convert from Kelvin to Celsius or Fahrenheit when using an Optical Pyrometer.

Back in the 1970s and before, people were willing to do that to get repeatable, accurate temperatures. Not any more! Beginning in the 1980s, detector-based, portable IR Thermometers came onto the market and changed the measurement areas forever. But, I am getting ahead.

Optical Pyrometers had a problem with spectral emissivity and people had a problem knowing which values to use. It wasn’t easy.

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